The invention relates to a rotary anode X-ray tube with an axial bearing arrangement which is particularly suitable for combination with radial spiral groove bearings.
An X-ray tube of this kind is known, for example from DE 30 17 291 and includes an anode (rotary anode) which is arranged so as to be rotatable within a vacuum tube and is connected to a motor rotor which is driven by a motor stator situated outside the tube. The anode is journalled radially by means of two spiral groove bearings and axially by means of two tooth-like structures of a magnetizable metal (iron) which are fixed to the rotary anode and a stationary part of the housing, respectively, the axial force effect being produced by cooperation of the teeth of the two structures which extend in the radial direction and are situated opposite one another. When the teeth are oriented so as to be situated exactly opposite one another, the axial force effect is zero. When the teeth are displaced relative to one another due to an axial motion of the rotary anode, the associated magnetic stray losses of the magnetic circuit cause an axial reset force at the area of the teeth. The magnet system thus constitutes a self-adjusting axial bearing with a stable zero point.
This magnet arrangement, however, has the drawback that at the same time, there are produced radial forces which are not self-adjusting and are even intensified outside the concentric central position of the anode. Such radial forces may lead to a high load and wear of the radial bearings notably when the rotary anode is switched on and off. This is notably so when spiral groove bearings are used as the radial bearings, because their bearing force is hardly existent yet at low speeds.
Therefore, it is an essential object of the invention to provide a rotary-anode X-ray tube with an axial bearing arrangement wherein the radial forces of the axial bearing are significantly smaller and which is particularly suitable for combination with radial spiral groove bearings.
This object is achieved in a first way by means of a rotary-anode X-ray tube of the kind set forth which is characterized in that the axial bearing arrangement includes a magnet arrangement with at least two elements which exert a magnetic force on one another, are spaced apart in the axial direction of the rotary anode and are connected in such a manner that the magnetic force takes up axial bearing forces in at least one direction.
As a result of the elements which are spaced apart in the axial direction of the rotary anode, said radial forces are substantially zero so that the loading of the radial bearing is significantly less.
It is an advantage of this solution that the magnet arrangement according to the invention has a construction which is significantly simpler and hence less expensive than the previously described tooth-like magnet structures. Moreover, in comparison with these magnet structures a significant amount of weight is saved, so that the loading of the radial bearings and their wear, notably during the starting and stopping phase, is proportionally less. Finally, the diameter of a rotary-anode bearing element or a bearing bush may be significantly larger, thus enabling the formation therein of one or more bores for cooling the anode by means of a cooling liquid.
A second way of achieving the above object involves a rotary-anode X-ray tube of the kind set forth which is characterized in that the axial bearing arrangement includes an end-face single-point ball bearing with a ball which lies in a cone and bears on a plate which acts as an abutment.
The dependent Claims relate to attractive further embodiments of the invention.
Another embodiment of the present invention offers the advantage that forces acting in both axial directions are taken up, so that the rotary anode can occupy a stable and force-free central position in the axial direction.
A particularly advantageous axial bearing arrangement of the present invention involves a combination of an axial magnet bearing and an axial end-face single-point ball bearing.